1. Field of the Invention
The present invention generally relates to a position measuring technical field using radio signals, and especially relates to a position measuring apparatus employing the GPS (Global Positioning System).
2. Description of the Related Art
In recent years and continuing, apparatuses for measuring positions utilizing the GPS are increasing. In general, the GPS is a system in which positions of two or more dimensions can be very precisely measured utilizing signals from at least three satellites out of about 24 satellites orbiting the earth. More specifically, triangulation type measurement, PHS type measurement, differential GPS type and other types of measurement are employed for measuring positions. These technologies are used for car navigation or in PDAs, and the positions of mobile stations or PDA's users are measured. The GPS utilizes radio waves emitted from satellites, and therefore satisfactory position measurement is not obtained in bad communication environments where the lines-of-sight between satellites and a position measuring apparatus (GPS receiver) are disturbed.
In this regard, conventionally Japanese Patent Laid-Open Publication 2000-111648 discloses a position measuring technology in which a position ID oscillator emitting infrared signals containing a position ID is provided indoors, a mobile station receives radio waves from satellites outdoors and receives the infrared signals indoors, and accordingly position measurement can be done outdoors and indoors.
Japanese Patent Laid-Open Publication 9-297171 discloses another technology in which a mobile object to be monitored is equipped with a detector for determining whether to receive signals from satellites, and accordingly it can be determined whether the object is located indoors or outdoors.
By the way, although position measurement techniques such as GPS provide highly precise position determination, errors of about 10 meters are normally unavoidable. Such errors cause no problem in wide space flatlands, but there exists a serious problem if an object to be monitored is on the boundary between a building and a road because there is a big difference between whether the object is on a roof of the building or on the road at the periphery of the building.
FIG. 1 is a schematic view illustrating positional variations of a mobile station, a building and measured coordinates. FIG. 1(A) is a top plan view seen from above a building, and FIG. 1(B) is a perspective view. It is assumed that a mobile station or its user 102 is actually located on a roof of the building 104, as seen in FIGS. 1(A) and 1(B). The position coordinates measuring is performed periodically at every one second, and measured coordinates 106 are obtained at every one second. However, the measured coordinates vary within a certain area 108. In FIG. 1, the variation area is indicated by a circle having a radius of several meters. This circle area includes two regions; one region is on the building 104 and the other region is on a road adjacent to the building 104. Accordingly it is unclear whether the mobile station 102 is located on the roof of the building or not, and therefore there is large uncertainty regarding the position of the object to be monitored.
With reference to FIG. 2 showing schematic views similar to FIG. 1, in a case where the mobile station 102 is actually on a road adjacent to the building 104, signals from some satellites are blocked off by the building. Accordingly the variation area becomes larger, because the signals utilized for measuring the position are limited, resulting in lower precision. Therefore, it is more uncertain whether the mobile station 102 is located on the roof of the building, than in FIG. 1.
Both of the conventional techniques disclosed in the above two Japanese Patent Documents determine whether a mobile station is located indoors or outdoors. However, a roof of a building and a road adjacent the building are both outdoors, and therefore it is difficult for those conventional techniques to determine whether a mobile station is located on the roof of the building or a road adjacent to the building.
On the other hand, it is theoretically possible to solve the above problem by measuring a position of a mobile station in a three-dimensional mode including height measurement. However, three dimensional measurement needs to receive satisfactory signals from four or more satellites. When more than three satellites cannot be captured (signals received satisfactorily), it is difficult to perform three-dimensional measurement. Further, even if more than three satellites can be captured, the measurement accuracy in the height direction is generally worse (for example, 1.5 times worse) than that in the horizontal direction. Therefore, it is difficult to determine whether a mobile station is located on a roof of a building having a height lower than 10 meters or not, although that information is of great practical use.
A DOP scheme for improving measurement accuracy is known, in which an amount called DOP (Dilution of Precision) is utilized to stop outputting unreliable measured coordinates. The DOP is a parameter that indicates the probabilities of measured coordinates based on geometric configurations of used satellites with regard to a mobile station. For example, if all the three satellites used for measuring position exist in similar directions, DOP has a large value, which means reliability of the position coordinates is low. By limiting measured coordinates having large DOPs and allowing only small DOP coordinates to be output, it becomes possible to output highly reliable measured coordinates only. However, when the mobile station 102 is on a road close to the building 104 as shown in FIG. 2, its DOP is generally large, and therefore it becomes impossible to output measured coordinates.
Accordingly, it is desirable to provide a position measuring apparatus that can measure the position of a mobile station very precisely using the GPS.